Low current wide VREF range input buffer

ABSTRACT

A low-current input buffer is disclosed. The buffer uses self-biased N and P channel differential pairs with their outputs tied together. The self-biasing assists in reducing current consumption. The combination of N and P-channel differential pairs results in symmetry across a wide range of reference and supply voltages.

This application is a divisional of application Ser. No. 10/161,601,filed on Jun. 5, 2002, now U.S. Pat. No. 6,864,725, which isincorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a low-current differential buffer that worksacross a wide range of reference and supply voltages.

BACKGROUND OF THE INVENTION

There is a need for differential buffers that have uniform, symmetricalrise and fall trigger characteristics. Bazes buffers sense bothlow-to-high and high-to-low transitions equally, symmetrically, and withhigh speed. However, Bazes buffers tend to consume a lot of current, andalso sometimes function inconsistently depending on the referencevoltage and Vcc supplied thereto. Therefore, a low-current symmetricalbuffer that can work across a wide range of reference and supplyvoltages is desired.

BRIEF SUMMARY OF THE INVENTION

In one aspect, the invention provides a lower power buffer in which alldifferential amplifiers contained therein are self-biased. Thedifferential amplifiers can be either fully or half self-biased, wherethe half self-biased embodiment consumes slightly more power butrequires less transistors to create. In another aspect, the buffer isdouble- rather than single-ended, which provides a more reliable outputand faster switching speed. Additional aspects of the present inventionlocate enable gates at a variety of positions within the buffer.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and other features and advantages of the invention willbecome more apparent from the detailed description of the exemplaryembodiments of the invention given below in connection with theaccompanying drawings in which:

FIG. 1 is a schematic diagram depicting a portion of an input bufferwith a resistor bias;

FIG. 2 is a schematic diagram depicting a portion of an input bufferwith a self-biasing transistor;

FIG. 3 is a schematic diagram depicting an differential input buffer;

FIG. 4 is a schematic diagram depicting a Bazes buffer;

FIG. 5 is a schematic diagram depicting a differential buffer biasedusing resistors to choke off current;

FIG. 6 is a schematic diagram depicting a buffer according to a firstembodiment of the present invention;

FIG. 7 is an additional schematic diagram of the buffer of FIG. 6, whileFIGS. 7A, 7B, 7C, 7D, and 7E show variations of the buffer of FIG. 7;

FIG. 8 is a schematic diagram of an additional embodiment of the presentinvention, while FIG. 8A, 8B, 8C, 8D, and 8E show variations of thatembodiment;

FIG. 9 is a schematic diagram of an additional embodiment of the presentinvention, while FIGS. 9A, 9B, and 9C show variations of thatembodiment;

FIG. 10 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 10A, 10B, 10C, 10D, and 10E showvariations of that embodiment;

FIG. 11 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 11A, 11B, and 11C show variations of thatembodiment;

FIG. 12 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 12A, 12B, 12C, and 12D show variations ofthat embodiment;

FIG. 13 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 13A, 13B, and 13C show variations of thatembodiment;

FIG. 14 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 14A, 14B, 14C, 14D, and 14E showvariations of that embodiment;

FIG. 15 is a schematic diagram of an additional embodiment of thepresent invention, while FIGS. 15A, 15B, 15C, 15D, and 15E showvariations of that embodiment;

FIG. 16 show the present invention as part of a processor system.

DETAILED DESCRIPTION OF THE INVENTION

Differential input buffer circuits are useful in digital circuits fordetermining whether an unknown input voltage V_(IN) is either above orbelow a fixed reference voltage V_(REF). Specifically, whenV_(IN)>V_(REF), a definite output is expected, and when V_(IN)<V_(REF),another, opposite output is expected. However, to guarantee that thedifferential input buffer circuit works properly, V_(IN) must differfrom V_(REF) by an offset of no less than a predetermined voltage, e.g.300 mV.

It is a necessary feature of differential buffer circuits to be biasedin order that the transistors contained or known therein will be inoperational mode at all times. FIG. 1 shows a portion of buffer circuit100 in which an n-channel differential amplifier 108 is biased using aresistor 104. This arrangement, however, has the disadvantage that it isdifficult to consistently fabricate resistors having the exact sameresistance value. Also, the resistance value of resistor 104 may notremain consistent all voltage and temperature ranges during operation,therefore impacting the circuit's bias stability.

FIG. 2 shows portion of another known buffer circuit 200 which is animprovement over FIG. 1, in that a portion of an input buffer circuit200 contains an n-channel differential amplifier 204 which isself-biased. As shown in FIG. 2, this self-biasing is accomplished byconnecting the source 208 of one of the ndiff transistors (n-channeltransistors within a differential amplifier circuit) to the gate 212 ofa biasing transistor 216. Thus, improved biasing and switching within abuffer circuit can be achieved.

FIG. 3 shows an n-channel differential pair 300, where the currentmirror portion 308 ensures that the amount of current through bothp-channel transistors is equal. The differential pair 300 is enabled bya biasing transistor 304. Substituting a transistor 304 for the resistor104 of FIG. 1 stabilizes some of the fabrication problems associatedwith resistor biasing, but is still tied to an unstable referenceresistance tree 316. The circuit 300 also does not operate symmetricallyin that it senses low-to-high transitions of V_(IN) differently than itsenses high-to-low transitions. This problem is exacerbated when thedifferential pair 300 is operated across a wide range of referencevoltages.

FIG. 4 is a schematic diagram depicting a Bazes buffer 400 employing acircuit using the self-biasing principles of FIG. 2. As shown in FIG. 4,the gates of p- and n-channel bias transistors 412, 416 are connected ata point between the p- and n-channel differential amplifiers 404, 408.P-channel transistor 412 is connected to Vcc, while n-channel transistor416 is connected to ground. With this biasing technique, the Bazesbuffer 400 responds to inputs in a more symmetrical fashion, but worksbest when V_(REF) is exactly Vcc/2. As V_(REF) varies from Vcc/2, thesymmetry is reduced.

FIG. 4 also shows that the sources of the transistors within thep-channel differential amplifier 404 are connected together at V_(H),and that the drains of the transistors within the n-channel differentialamplifier 408 are connected together at V_(L). In order for the circuitof FIG. 4 to be biased in a stable fashion, the currents throughtransistors 412 and 416 steer the current to enable stable switching ofthe output based on the Vbias level. When Vbias goes higher, then-channel supplies more current to the ground side. As Vbias goes lower,the p-channels provide more current to the Vcc side. The twobias-voltage inputs are connected to the internal amplifier bias nodeV_(BIAS). This self-biasing creates a negative-feedback loop thatstabilizes the bias voltages. Additionally, any variations in processingparameters or operating conditions that shift the bias voltages awayfrom their nominal values result in a shift in V_(BIAS) which correctsthe bias voltages through negative feedback.

In the Bazes buffer of FIG. 4, transistors 412 and 416 operate in thelinear region. Consequently, the voltages V_(H) and V_(L) may be setvery close to the supply voltages. Since these two voltages determinethe output swing of the amplifier, the output swing can be very close tothe difference between the two supply rails. This large output swingsimplifies interfacing the Bazes amplifier to other types of logicgates, since it provides a large margin for variations in the logicthreshold of the gates.

Another consequence of the linear range operation of transistors 412 and416 is that the Bazes amplifier 400 can provide output switchingcurrents which are significantly greater than its quiescent current. Incontrast, conventional CMOS differential amplifiers cannot provideswitching currents which exceed the quiescent current set by thecurrent-source drive, which operates in the saturation region. Thiscapability of supplying momentarily large current pulses makes the Bazesamplifier especially suitable for high-speed comparator applicationswhere one of the inputs V_(IN) is a reference voltage, and where it isnecessary to rapidly charge and discharge output capacitive loadswithout consuming inordinate amounts of power. Bazes buffers such asthat shown in FIG. 4 detect transitions faster, symmetrically, and withless distortion than other buffers. These are very desirablecharacteristics for high speed data buffers such as input/output buffersfor memory devices and processors. Unfortunately, Bazes buffers have thedisadvantage that they consume a large amount of current. Also, asV_(REF) moves away from Vcc/2, Bazes buffers lose stability due tocurrent mismatches, because rise and fall times do not track as well asdesired.

FIG. 5 shows a known buffer 500. Resistors 504 and 508 assist inachieving bias stability, but as noted with respect to FIG. 1 it isdifficult to consistently fabricate resistors having the exact sameresistance value. Also, the current supplied through those resistorsvalue may not remain consistent during operation, therefore impactingthe circuit's bias stability especially across a range of values forVcc. Additionally, the buffer 500 still has the problem of consumingexcessive amounts of current, as well as inconsistent performance acrossa range of reference voltages. For example, when V_(REF) is low, thebuffer 500 maintains fairly good symmetry but consumes a lot of current.When V_(REF)=Vcc/2 which is the generally accepted optimum condition ofoperation, timing data regarding both output conditions is gathered atthe output node 512. This data shows that rise time T_(rise) (the resultwhen V_(IN)>V_(REF)) is faster than fall time T_(fall) (the result whenV_(IN)<V_(REF)), which causes the buffer 500 to behave asymmetrically.Finally, as V_(REF) increases, the buffer 500 continues to have its biasnodes 516, 520 go lower, yet the output node 512 stays very close toground. This results in the drive ratios of the combined differentialamplifiers 524 and 528 becoming skewed so that the buffer ceases toswitch well, because T_(fall) increases while T_(rise) decreases.Accordingly, the response again becomes asymmetrical. Even worse, asV_(REF) continues to increase, the P-channel amplificationcharacteristics become so strong that the buffer ceases to work at all.Thus, the drive ratios necessary to make the buffer work well at thelower V_(REF)s inhibit the buffer's performance at the higher V_(REF)s.

These problems are mitigated by the input buffers of the presentinvention as shown in the following Figures. In FIG. 6, the input buffer600 has two transistors 604, 608 for the p-differential amplifier 620,and two other transistors 612, 616 for the n-differential amplifier 624.The transistors 604, 608, 612, and 616 are included specifically for thepurpose of self-biasing, which provides for more consistent performanceacross a wide range of reference voltages. These transistors result inboth p- and n-differential amplifiers 620, 624 being fully self-biased.In contrast, the known circuit of FIG. 5 shows both p- andn-differential amplifiers 524, 528 not self-biased, but instead beingchoked by resistors 504 and 508. These current-choking resistors 504,508 cause the differential amplifiers 524, 528 to consume less current,but do not produce a measurable biasing effect. The advantage of beingself-biased is that, as V_(REF) increases, bias nodes 628, 632 stayfairly constant, perhaps increasing slightly. During this time then-channel node 636 node greatly increases, yet the drive ratios of thep-channel to n-channel amplifiers do not change. It is well known that ap-channel differential amplifier works better at lower referencevoltages, while n-channel differential amplifiers work better at higherreference voltages. The present invention combines these twocharacteristics while minimizing the disadvantages associated therewith.This enables the buffers of the present invention, including all of thefollowing embodiments, to work with either synchronous dynamic randomaccess memory (SDRAM), double data rate random access memory (DDRRAM),and low power mobile random access memory (LPMRAM), all of which requirediffering reference voltages.

FIG. 7 shows the circuit of FIG. 6 reworked to fit horizontally on apage rather than vertically, and also with the addition of “enable gate”transistors 704 and 708. The enable gate transistor 704 couples biasingtransistor 604 and 612 to VCC, while enable gate transistor 708 couplesbiasing transistor 608 and 616 to ground. Enable gates allow inputbuffers such as the buffer 600 and 700 to be disabled to save current incertain operating modes. Various embodiments of the present inventionwill be shown in which portions of the n- and p-channel differentialamplifiers either share enable gates or have their own enable gates.Sharing enable gates allow the buffers to consume less current andassist the bias node in tracking variances in the reference voltageV_(REF) and then making any necessary drive ratio adjustments resultingfrom those variances, thereby resulting in more consistent rise and falltimes. The buffer of FIG. 7 works reliably with an input voltage V_(IN)and a reference voltage V_(REF), but also can work with complementaryV_(IN) and V_(IN−) inputs.

Additionally, it is important to note that all of the followingembodiments have the output side of the buffer is tied to the side ofthe differential pair that is close to the input signal V_(IN), not thereference signal V_(REF). In this way, the output of the buffer isalways inverting. However, the present invention could also be arrangedto have the output side of the buffer tied to the side of thedifferential pair that is close to the reference signal V_(REF), not theinput signal V_(IN). This arrangement would always be noninverting.

Additional variations of the FIG. 7 circuit contemplated by theinvention include replacing one of the two biasing transistors 604 and608 with a direct connection between buffer 620 and transistor 704 andbuffer 620 and enable gate 708 respectively, which would then make thep-differential amplifier 620 only half self-biased, while then-differential amplifier 624 remains fully self-biased. Similarly, oneof the two biasing transistors 612 and 616 could also be replaced with adirect connection between amplifier 624 and either enable gate 704 or708, which would then make the n-differential amplifier 624 only halfself-biased, while the p-differential amplifier 620 would remain fullyself-biased.

FIG. 7 shows both p- and n-differential amplifiers 620, 624 respectivelysharing enable gates 704 and 708. However, each differential amplifiercould have its own enable gate 704′ or 708′ (for p-differentialamplifier 620) and 704″, 708″ (for n-differential amplifier 624), asshown in FIG. 7A. Additionally, each differential amplifier 620, 624could share one enable gate 704 as in FIG. 7, but also have separateenable gates 708′, 708″ instead of enable gate 708 as in FIG. 7A, orvice versa.

As another variation to the FIG. 7 circuit, one of the enable gates 704,708 but not both could be replaced by a resistor 704 b or 708 c, asshown in FIGS. 7B and 7C. Furthermore, either enable gate 704 or 708 ofFIG. 7 could be replaced by series transistors. In the case where theenable gate 704 is replaced by p-channel series transistors, thetransistors would have their gates tied to V_(A) as shown in FIG. 7D,where ground <=V_(A)<V_(TP) (threshold voltage of the p-channeltransistors). In the case where the enable gate 708 is replaced byn-channel series transistors, the transistors would have their gatestied to V_(B) as shown in FIG. 7E, where threshold voltage of then-channel transistors V_(TN)<V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 8, in whicha differentially folded input buffer 800 has mirrored p-differentialamplifiers 804, 808, as well as mirrored n-differential amplifiers 812,816. As shown in FIG. 8, the outputs of differential amplifiers 804 and812 are combined, and the outputs of differential amplifiers 808 and 816are combined. The amplifiers 804, 812 are coupled to provide thedifferential output V_(OUT), while the amplifiers 808, 816 are combinedto provide the complementary signal V_(OUT−). The FIG. 8 circuit isadvantageous for improving reliability and accuracy of the two outputsignals, and also for creating two complementary outputs as opposed to asingle ended output.

The FIG. 8 embodiment has the differential amplifiers 804, 808 fullyself-biased by transistors 820, 824, and the differential amplifiers812, 816 filly self-biased by the transistors 828 and 832. FIG. 8 alsohas enable gates 836 and 840, which couple bias transistors 820 and 828to Vcc and bias transistors 824, 832 to ground, respectively.

Variations of the FIG. 8 circuit which also form part of the inventioninclude replacing either one of the two biasing transistors 820 and 824with a direct connection to enable gate 836 or 840, which would thenmake the mirrored p-differential amplifiers 804, 808 only halfself-biased, while the mirrored n-differential amplifiers 812, 816 wouldremain fully self-biased. Similarly, one of the two biasing transistors828 and 832 could also be replaced with a direct connection to enablegate transistor 836 or enable transistor 840, which would then make themirrored n-differential amplifiers 828, 832 only half self-biased, whilethe mirrored p-differential amplifiers 820, 824 would remain fullyself-biased.

FIG. 8 shows all mirrored differential amplifiers 804, 808, 812, 816sharing enable gates 836 and 840. However, each set of 804, 808 mirroreddifferential amplifiers (804, 808 and 812, 816) could have its ownenable gates 836′, 840′ and 836″, 840″ as shown in FIG. 8A.Additionally, each mirrored differential amplifier 804, 808, 812, 816and FIG. 8 could share enable gate 836 as shown in FIG. 8, but haveseparate enable gates 840′, 840″ as shown in FIG. 8A instead of enablegate 840, or vice versa.

Furthermore, one but not both of the enable gates 836, 840 of FIG. 8could be replaced by a resistor 836 b, 840 c or series transistors 836d, 840 e as shown in FIG. 8B-8E. In the case where the enable gate 836to Vcc of FIG. 8 is replaced by p-channel series transistors 836 d (FIG.8D), the transistors would have their gates tied to V_(A), where ground<=V_(A)<V_(TP). In the case where the enable gate 840 to ground of FIG.8 is replaced by n-channel series transistors 840 e (FIG. 8E), thetransistors would have their gates tied to V_(B), whereV_(TN)=V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 9, in whichan input buffer 900 has a p-differential amplifier 904 as well as ann-differential amplifier 908. Both amplifiers 904, and 908, however, areonly half self-biased, as shown by the dashed circles 909 and 910 whichare meant to draw attention to the lack of an electronic componentlocated there. Being half self-biased is advantageous in that fewertransistors are used, but results in slightly higher currentconsumption. As stated, all of the embodiments of the present inventionusing half rather than fill self-biasing have the disadvantage ofslightly higher current consumption. However, response symmetry is notmeasurably affected by half self-biasing. Both amplifiers 904 and 908share a p-channel enable gate 914 to Vcc.

Additional variations of the FIG. 9 embodiment could include eachdifferential amplifier 904 and 908 having its own enable gate 914′,914″, as shown in FIG. 9A. Furthermore, the enable gate 914 of FIG. 9could be replaced by a resistor 914 b or series transistors 914 c withtheir gates tied to V_(A) where ground <=V_(A)<V_(TP), as shown in FIGS.9B and 9C, respectively.

Another embodiment of the present invention is shown in FIG. 10, whereina p-differential amplifier 1004 is half self-biased, while ann-differential amplifier 1008 is fully self-biased. Both amplifiers 1004and 1008 share an n-channel enable gate 1012 to ground as well as ap-channel enable gate 1016 to Vcc. This embodiment, like all of theembodiments having half rather than fill self-biasing, has thedisadvantage of slightly higher current consumption yet requires lesstransistors and maintains satisfactory response symmetry.

Additional variations of the FIG. 10 embodiment could include eachdifferential amplifier 1004, 1008 having its own enable gates 1016′,1016″, 1012′, 1012″, as shown in FIG. 10A. Additionally, eachdifferential amplifier could share enable gate 1016 as in FIG. 10, buthave separate enable gates 1012′, 1012″ as shown in FIG. 10A, or viceversa. Furthermore, either (but not both) enable gates 1012, 1016 ofFIG. 10 could be replaced by a resistor 1004 b, 1008 c or seriestransistors 1004 d, 1008 e as shown in FIGS. 10B-10E. In the case wherethe enable gate to Vcc 1016 of FIG. 10 is replaced by p-channel seriestransistors 1016 d (FIG. 10D), the transistors would have their gatestied to V_(A), where ground <=V_(A)<V_(TP). In the case where the enablegate 1012 to ground of FIG. 10 is replaced by n-channel seriestransistors 1012 e (FIG. 10E), the transistors would have their gatestied to V_(B), where V_(TN)<V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 11, whereina p-differential amplifier 1104 is fully self-biased, while ann-differential amplifier 1108 is also fully self-biased. Both amplifiers1104 and 1108 share an n-channel enable gate 1112 to ground. However,the bias transistors 820, 828 are directly tied to Vcc.

Additional variations of the FIG. 11 embodiment could include replacingany one of the four biasing transistors 820, 828, 824, 832 with a directconnection to Vcc (for replaced transistors 820, 828) or enable gatetransistor 1112 (for transistors 824, 832), which would then make one ofthe two differential amplifiers 1104 and 1108 half self-biased.

Additionally, FIG. 11 shows both p- and n-differential amplifiers 1104,1108 sharing the enable gate 1112. However, each differential amplifiercould have its own enable gate 1112′, 1112″ as shown in FIG. 11A.Furthermore, enable gate 1112 of FIG. 11 could be replaced by a resistor1112 b or series transistors 1112 c, as shown in FIGS. 11B and 11Crespectively. In the case where the enable gate 1112 to ground isreplaced by n-channel series transistors 1112 c, the transistors wouldhave their gates tied to V_(B), where V_(TN)<V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 12, whereina p-differential amplifier 1204 is fully self-biased, and ann-differential amplifier 1208 is also fully self-biased. Both amplifiers1204 and 1208 have their own enable gates to both Vcc (1205, 1209) andto ground (1206, 1210).

Additional variations of the FIG. 12 embodiment could include replacingany one of the biasing transistors 1220, 1221, 1222, 1223 with a directconnection, which would then make one of the differential amplifiersonly half self-biased, while the other would remain fully self-biased.Furthermore, one or more of the enable gates 1205, 1206, 1209, 1210could be replaced by a resistor or series transistors, as shown in FIGS.12A, 12B for replacing one or both of transistor enable gate 1206, 1209and FIGS. 12C, 12D for replacing one or both of enable gate transistors1205, 1210. In the case where an enable gate to Vcc is replaced byp-channel series transistors, the transistors would have their gatestied to ground. In the case where an enable gate to ground is replacedby n-channel series transistors, the transistors would have their gatestied to V_(B), where V_(TN)<V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 13, whereina p-differential amplifier 1304 is fully self-biased by transistors1320, 1321, and an n-differential amplifier 1308 is also fullyself-biased by transistor 1322, 1323. The amplifiers 1304 and 1308 sharea p-channel enable gate 1312 to Vcc while bias transistors 1321, 1323are tied to ground.

Additional variations of the FIG. 13 embodiment include replacing one ofthe biasing transistors 1320, 1321, 1322, and 1323 with a directconnection, which would then make one of the differential amplifiers1304, 1308 only half self-biased, while the other would remain fullyself-biased. Additionally, FIG. 13 shows both differential amplifiers1304, 1308 sharing the enable gate 1312. However, each differentialamplifier could have its own enable gate, 1312′, 1312″ as shown in FIG.13A. Furthermore, the enable gate 1312 could be replaced by a resistor1312 b (FIG. 13B) or series transistors 1312 c (FIG. 13C) with theirgates tied to V_(A), where ground <=V_(A)<V_(TP).

Another embodiment of the present invention is shown in FIG. 14, whereina p-differential amplifier 1404 is fully self-biased, while ann-differential amplifier 1408 is half self-biased. The amplifiers 1404and 1408 share a p-channel enable gate 1412 to Vcc and an n-channelenable gate 1416 tied to ground.

Additional variations of the FIG. 14 embodiment include eachdifferential amplifier having its own enable gate 1412′, 1412″, 1416′,and 1416″ as shown in FIG. 14A. Additionally, each differentialamplifier 1464, 1408 could share enable gate 1412 of FIG. 14, but haveseparate enable gates 1416′, 1416″ instead of enable gate 1416 as inFIG. 14A, or vice versa. Furthermore, either (but not both) enable gates1412, 1416 of FIG. 14 could be replaced by a resistor or seriestransistors, as shown in FIGS. 14B and 14D for enable gate 1412, andFIGS. 14C and 14E for enable gate 1416. In the case where the enablegate 1412 to Vcc is replaced by p-channel series transistors, thetransistors would have their gates tied to V_(A), where ground<=V_(A)<V_(TP). In the case where the enable gate 1416 to ground isreplaced by n-channel series transistors (FIG. 14E), the transistorswould have their gates tied to V_(B), where V_(TN)<V_(B)<=Vcc.

Another embodiment of the present invention is shown in FIG. 15, whereina p-differential amplifier 1504 is fully self-biased by transistors1520, 1521, and an n-differential amplifier 1508 is also fullyself-biased by transistor 1521, 1523. The amplifiers 1504 and 1508 sharean n-channel enable 1512 gate to ground, while the n-differentialamplifier 1508 has its own separate p-channel enable gate 1516 to Vcc.

Additional variations of the FIG. 15 embodiment include replacing one ofthe biasing transistors 1520, 1521, 1522, and 1523 of FIG. 15 with adirect connection, which would then make one of the differentialamplifiers 1504, 1508 only half self-biased, while the other wouldremain fully self-biased. Additionally, FIG. 15 shows both differentialamplifiers 1504, 1508 sharing enable gate 1512. However, eachdifferential amplifier could have its own enable gate 1512′, 1512″, asshown in FIG. 15A. Furthermore, the enable gate 1516 of FIG. 15 could bereplaced by a resistor 1516 b (FIG. 15B) or series transistors 1516 d(FIG. 15D) with their gates tied to V_(A), where ground <=V_(A)<V_(TP).Additionally, the enable gate 1512 of FIG. 15 could be replaced by aresistor 1512 c (FIG. 15C) or series, transistors 1512 e (FIG. 15E) withtheir gates tied to V_(B), where V_(TN)<V_(B)<=Vcc.

The present invention can be utilized within any integrated circuitwhich receives an input signal from an external source. FIG. 16illustrates an exemplary processing system 1600 which may utilize anelectronic device comprising a self-biasing buffer constructed inaccordance with any of the embodiments of the present inventiondisclosed above in connections with FIG. 6 through 15E. The processingsystem 1600 includes one or more processors 1601 coupled to a local bus1604. A memory controller 1602 and a primary bus bridge 1603 are alsocoupled the local bus 1604. The processing system 1600 may includemultiple memory controllers 1602 and/or multiple primary bus bridges1603. The memory controller 1602 and the primary bus bridge 1603 may beintegrated as a single device 1606.

The memory controller 1602 is also coupled to one or more memory buses1607. Each memory bus accepts memory components 1608 which include atleast one memory device 1631 contains a buffer device of the presentinvention. The memory components 1608 may be a memory card or a memorymodule. Examples of memory modules include single inline memory modules(SIMMs) and dual inline memory modules (DIMMs). The memory components1608 may include one or more additional devices 1609. For example, in aSIMM or DIMM, the additional device 1609 might be a configurationmemory, such as a serial presence detect (SPD) memory. The memorycontroller 1602 may also be coupled to a cache memory 1605. The cachememory 1605 may be the only cache memory in the processing system.Alternatively, other devices, for example, processors 1601 may alsoinclude cache memories, which may form a cache hierarchy with cachememory 1605. If the processing system 1600 include peripherals orcontrollers which are bus masters or which support direct memory access(DMA), the memory controller 1602 may implement a cache coherencyprotocol. If the memory controller 1602 is coupled to a plurality ofmemory buses 16016, each memory bus 16016 may be operated in parallel,or different address ranges may be mapped to different memory buses1607.

The primary bus bridge 1603 is coupled to at least one peripheral bus1610. Various devices, such as peripherals or additional bus bridges maybe coupled to the peripheral bus 1610. These devices may include astorage controller 1611, an miscellaneous I/O device 1614, a secondarybus bridge 1615, a multimedia processor 1618, and an legacy deviceinterface 1620. The primary bus bridge 1603 may also coupled to one ormore special purpose high speed ports 1622. In a personal computer, forexample, the special purpose port might be the Accelerated Graphics Port(AGP), used to couple a high performance video card to the processingsystem 1600. In addition to memory device 1631 which may contain abuffer device of the present invention, any other data input device ofFIG. 16 may also utilize a buffer device of the present inventionincluding the CPU 1601.

The storage controller 1611 couples one or more storage devices 1613,via a storage bus 1612, to the peripheral bus 1610. For example, thestorage controller 1611 may be a SCSI controller and storage devices1613 may be SCSI discs. The I/O device 1614 may be any sort ofperipheral. For example, the I/O device 1614 may be an local areanetwork interface, such as an Ethernet card. The secondary bus bridgemay be used to interface additional devices via another bus to theprocessing system. For example, the secondary bus bridge may be anuniversal serial port (USB) controller used to couple USB devices 1617via to the processing system 1600. The multimedia processor 1618 may bea sound card, a video capture card, or any other type of mediainterface, which may also be coupled to one additional devices such asspeakers 1619. The legacy device interface 1620 is used to couple legacydevices, for example, older styled keyboards and mice, to the processingsystem 1600. In addition to memory device 1631 which may contain abuffer device of the invention, any other data input device of FIG. 16may also utilize a buffer device of the invention, including a CPU 1601.

The processing system 1600 illustrated in FIG. 16 is only an exemplaryprocessing system with which the invention may be used. While FIG. 16illustrates a processing architecture especially suitable for a generalpurpose computer, such as a personal computer or a workstation, itshould be recognized that well known modifications can be made toconfigure the processing system 1600 to become more suitable for use ina variety of applications. For example, many electronic devices whichrequire processing may be implemented using a simpler architecture whichrelies on a CPU 1601 coupled to memory components 1608 and/or memorybuffer devices 304. These electronic devices may include, but are notlimited to audio/video processors and recorders, gaming consoles,digital television sets, wired or wireless telephones, navigationdevices (including system based on the global positioning system (GPS)and/or inertial navigation), and digital cameras and/or recorders. Themodifications may include, for example, elimination of unnecessarycomponents, addition of specialized devices or circuits, and/orintegration of a plurality of devices.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as limited by the foregoing description but is onlylimited by the scope of the appended claims.

1. A differential input buffer, comprising: a buffer circuit havingseparate p- and n-channel differential amplifiers arranged to eachreceive a pair of input signals, said p- and n-channel differentialamplifiers each being self-biased and having a current mirror portion; aresistive circuit, at least one of said p- and n-channel differentialamplifiers being coupled to said resistive circuit; and an outputterminal for combining the outputs of said p- and n-channel differentialamplifiers to form an output of said differential input buffer, saidoutput terminal being coupled to a pair of series transistors, whereinsaid p- and n-channel differential amplifiers are coupled to a sharedp-channel enable transistor.
 2. The input buffer of claim 1, whereinsaid resistive circuit is coupled to ground.
 3. The input buffer ofclaim 1, wherein said resistive circuit is coupled to a supply voltage.4. The input buffer of claim 1, wherein: said resistive circuit iscoupled to a supply voltage.
 5. The input buffer of claim 1, wherein:said resistive circuit is coupled to a ground terminal; and said enabletransistor is coupled to a supply voltage terminal.
 6. A processingsystem circuit, comprising: a processor; and a memory circuit forexchanging data with said processor; wherein at least one of saidprocessor and memory circuit comprise an input buffer circuit, saidinput buffer circuit further comprising: a buffer circuit havingseparate p- and n-channel differential amplifiers, wherein said separatep- and n-channel differential amplifiers are each self-biased and havinga current mirror portion; and a resistive circuit, at least one of saidp- and n-differential amplifiers being coupled to said resistivecircuit, wherein the outputs of said differential amplifiers arecombined to form an output of said differential input buffer, saidoutput being coupled to a pair of series transistors, and wherein saidp- and n-channel differential amplifiers are coupled to a sharedp-channel enable transistor.
 7. The processor circuit of claim 6,wherein said resistive circuit is coupled to a ground terminal.
 8. Theprocessor circuit of claim 6, wherein said resistive circuit is coupledto a supply voltage terminal.
 9. The processor circuit of claim 6,wherein said resistive circuit is coupled to ground.
 10. The processorcircuit of claim 6, wherein said resistive circuit is coupled to asupply voltage terminal.
 11. The processor circuit of claim 6, wherein:said resistive circuit is coupled to a ground terminal, and said sharedp-channel enable transistor is coupled to a supply voltage terminal. 12.The processor circuit of claim 6, wherein said resistive circuit iscoupled to a ground terminal.
 13. The processor circuit of claim 6,wherein said resistive circuit is coupled to a supply voltage terminaland wherein said p- and n-channel differential amplifiers are eachdirectly connected to a ground terminal.
 14. The processor circuit ofclaim 6, wherein: said resistive circuit is coupled to a supply voltageterminal.
 15. A method of operating an input buffer, comprising:self-biasing a pair of separate p- and n-channel differentialamplifiers, each of said differential amplifiers having a current mirrorportion; enabling at least one of said p- and n-channel differentialamplifiers, wherein said p- and n-differential amplifiers are coupled toa resistive circuit and to at least one pair of series transistors at anoutput node; detecting an input signal; comparing said input signal witha reference signal using said separate p- and n-channel differentialamplifiers; and emitting the results of said comparison as an outputsignal, wherein said p- and n-channel differential amplifiers arecoupled to a shared p-channel enable transistor.
 16. The method of claim15, wherein said resistive circuit is coupled to ground.
 17. The methodof claim 15, wherein said resistive circuit is coupled to a voltagesupply.
 18. The method of claim 15, wherein said resistive circuit iscoupled to ground.
 19. The method of claim 15, wherein said resistivecircuit is coupled to a supply voltage.
 20. The method of claim 15,wherein: said resistive circuit is coupled to ground, and said sharedenable transistor is coupled to a supply voltage.
 21. The method ofclaim 15, wherein said resistive circuit is coupled to a supply voltage.